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I'm attempting to encode a String in a client using GZIPOutputStream then decoding the String in a server using GZIPOutputStream.

The client's side code (after the initial socket connection establishment) is: 

// ... Establishing connection, getting a socket object.
// ... Now proceeding to send data using that socket:

DataOutputStream out = new DataOutputStream(socket.getOutputStream());
String message = "Hello World!";

ByteArrayOutputStream out = new ByteArrayOutputStream();
GZIPOutputStream gzip = new GZIPOutputStream(out);
gzip.write(message);
gzip.close();
String encMessage = out.toString();

out.writeInt(encMessage.getBytes().length);
out.write(encMessage.getBytes());
out.flush();

And the server's side code (again, after establishing a connection):

DataInputStream input = new DataInputStream(socket.getInputStream());

int length = input.readInt();
byte[] buffer = new byte[length];
input.readFully(buffer);

GZIPInputStream gz = new GZIPInputStream(new ByteArrayInputStream(buffer));
BufferedReader r = new BufferedReader(new InputStreamReader(gz));
String s = "";
String line;
while ((line = r.readLine()) != null) 
{
    s += line;
}

I checked and the buffer length (i.e., the coded message's size) is passed correctly, so the right number of bytes is transferred. However, I'm getting this:

java.util.zip.ZipException: invalid code lengths set
at java.util.zip.InflaterInputStream.read(InflaterInputStream.java:164)
at java.util.zip.GZIPInputStream.read(GZIPInputStream.java:117)
at java.util.zip.InflaterInputStream.read(InflaterInputStream.java:122)
at parsing.ReceiveResponsesTest$TestReceiver.run(ReceiveResponsesTest.java:147)
at java.lang.Thread.run(Thread.java:745)

Any ideas?

Thanks in advance for any assistance!

amirkr
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    Try gzip.finish() before gzip.close() at client side. GZIP requires some ending bytes at the end of stream. – Eugene Feb 06 '17 at 08:21
  • Thx Eugene, eventually only using a byte stream instead of coding into a string and then decoding that string worked :) – amirkr Feb 06 '17 at 17:36

2 Answers2

4

You're calling toString() on the ByteArrayOutputStream - that is incorrect, and it opens up all kinds of character encoding problems that are probably biting you here. You need to call toByteArray instead:

byte[] encMessage = out.toByteArray();

out.writeInt(encMessage.length);
out.write(encMessage);

Detail:

if you use toString(), Java will encode your bytes in your platform default character encoding. That could be some Windows codepage, UTF-8, or whatnot. However not all characters can be encoded properly, and some will be replaced by an alternative character - a question mark perhaps. Without knowing the details, it's hard to tell.

But in any case, encoding the byte array to a String, and then decoding it to a byte array again when you write it out, is very likely to change the data in the byte array. And there is not need to do it, you can just get the byte array straight away as shown in the code above.

Erwin Bolwidt
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Why on earth are you indulging in all this complication? You can reduce it all to this:

GZIPOutputStream gzip = new GZIPOutputStream(socket.getOutputStream());
DataOutputStream out = new DataOutputStream(gzip);
String message = "Hello World!";    
out.writeUTF(message);
out.close();

// ...    

GZIPInputStream gz = new GZIPInputStream(new ByteArrayInputStream(socket.getInputStream()));
DataInputStream input = new DataInputStream(gz);
String line = input.readUTF();

I further note that your code doesn't actually compile. I would further note that unless the messages are several orders of magnitude larger, there is no benefit to the GZipping.

user207421
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  • Hi, thanks - sorry if the code does not compile "as-is", I copied the client's code from a different computer manually. Also the message string is an example one, the actual messages are above 15KB which makes the compression count. Thanks for your reply! – amirkr Feb 06 '17 at 15:33